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Alkyne reactions
Organic Chemistry MCAT
| Question | Answer |
|---|---|
| What are the trends for the boiling points of alkynes? | Alkynes boil at a higher temperature than alkenes because of the higher polarity (due to asymmetrical distribution of electrons). Internal alkynes boil at a higher temperature than terminal alkynes. |
| Why are terminal alkynes acidic? | They are fairly acidic (weak compared to alkenes), but can stabilize a negative charge. |
| How can we synthesize alkynes? | Two rounds of elimination of an alkyl dihalide in the presence of a strong base and heat. Also, a terminal alkyne is made nucleophilic by removing a proton with help of a strong base. The nucelophile can displace an alkyl halide to form an internal alkyne |
| What are the reactions of alkynes? | Reduction, additions (electrophilic and free radical), hydroboration, and oxidation. |
| Explain the reduction of alkynes? | Reduction of an alkyne using the lindlar catalyst results in a cis- alkene while reduction with a sodium or lithium in ammonia results in a trans-alkene |
| Explain the electrophilic addition of alkynes? | An alkyne can react with one molecule of bromine to form a dihalide alkene. When an alkyne reacts with two molecules of bromine, an alkane is formed. |
| Explain the free radical reaction of alkynes? | As similar to alkenes, it is an anti-markovnikov reaction. The halide radical will attach to the least substituted carbon, and forms a trans-isomer (alkene) |
| Explain the hydroboration of internal alkynes? | As similar to alkenes, it follows an anti-markovnikov orientation. But, there is no hydrogen peroxide used. When the resulting product is washed with acetic acid, a cis alkene is formed. |
| Explain the hydroboration of terminal alkynes? | The terminal alkynes will react in presence of hydrogen peroxide, and forms an enol (alkene-alcohol), and then they will rearrange to form an aldehyde. |
| Explain the oxidation of alkynes? | The alkynes (internal) will react with the hot, KMNO4, and form two molecules of carboxylic acid. The internal alkyne will react with ozone and hydrogen peroxide to form two moles of carboxylic acids. But, for terminal alkyne produce carboxylic acid & CO2 |
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prijuvarghese
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